Microwave powered reusable orbiting space tug

ABSTRACT

This space vehicle is used as a &#39;&#39;&#39;&#39;tugboat&#39;&#39;&#39;&#39; for propelling other space vehicles. The tug has a pair of propulsion nozzles to which a propulsion fluid is fed by way of an absorption chamber. A large microwave antenna is mounted on the space tug for receiving and concentrating a microwave beam which may come from the earth&#39;&#39;s surface. The nozzles and antenna are pivotable relative to each other. Large but short wave guides lead from the feed horn of the antenna through the pivot trunnions for conveying the concentrated microwave beam to the absorption chambers. The beam, which to this point has travelled through a vacuum, is nearly quantitively absorbed by the propulsion fluid which is thereby heated to a plasma. The plasma is directed to the propulsion nozzle by a magnetic field. A single component propulsion fluid is contained in replaceable tanks and energy is imparted to the fluid by way of the microwave beam rather than by chemical reaction. A phased array of antennas permits focusing at high orbital altitudes.

United States Patent 11 1 Minovitch MICROWAVE POWERED REUSABLE ORBITINGSPACE TUG [76] Inventor: Michael A. Minovitch, 2832 St.

George St, Los Angeles. Calif. 90027 [22] Filed: Mar. 21, 1973 [21]Appl. No.: 343,197

1521 11.5.0 ..244/1ss;244/15 B; 244/62 151 1111.131. 864g 1/00 158 Fieldof Search 244/1 55, 1 SA, 1 SB, 62,

OTHER PUBLICATIONS Alexander, George, Major Space Role Seen for PlasmaEngine Aviation Week and Space Technology, Nov. 27. 1961, pp. 75-76.

1451 June 24, 1975 Primary Examiner-Trygve M. Blix AssistantExaminer-Barry L. Kelmachter Attorney, Agent, or Firm-Christie. Parker &Hale l 5 7 ABSTRACT This space vehicle is used as a tugboat" forpropelling other space vehicles. The tug has a pair of propulsionnozzles to which a propulsion fluid is fed by way of an absorptionchamber. A large microwave antenna is mounted on the space tug forreceiving and concentrating a microwave beam which may come from theearths surface. The nozzles and antenna are pivotable relative to eachother. Large but short wave guides lead from the feed horn of theantenna through the pivot trunnions for conveying the concentratedmicrowave beam to the absorption chambers. The beam, which to this pointhas travelled through a vacuum, is nearly quantitively absorbed by thepropulsion fluid which is thereby heated to a plasma. The plasma isdirected to the propulsion nozzle by a magnetic field. A singlecomponent propulsion fluid is contained in replaceable tanks and energyis imparted to the fluid by way of the microwave beam rather than bychemical reaction. A phased array of antennas permits focusing at highorbital altitudes.

10 Claims, 4 Drawing Figures PATENTEDJUM24 ms .891; 16G

SHEET 1 MICROWAVE POWERED REUSABLE ORBITING SPACE TUG BACKG ROUN D Inpresent space flight operations few, if any, components are reusable,although a reusable space shuttle is in development. Chemically poweredrockets are used to boost payloads into earth orbit and these payloadsare often boosted into escape trajectories by chemical propulsion. Theescape trajectory boosters typically are lost into space and are notreusable. Presently, all orbital maneuvering is by chemical propulsion.Thus, the propulsion fluid and energy for heating it are put into orbitby chemically propelled boosters.

It is desirable to use a low molecular weight propulsion fluid such ashydrogen for high impulse, however, in chemical boosters, the hydrogenis typically combined with oxygen which has a significantly greaterdensity and molecular weight. Energy can be supplied to an orbitingvehicle by an electro-magnetic beam, and this high energy beam used forheating a single propellant component, such as hydrogen, which can beused as the propulsion fluid without any chemical reactions.

It has been proposed to use beamed microwave power for space propulsion,thus, in a paper entitled Microwave Powered Ferry Vehicles inspaceflight (June 1966}, page 217, M. I. Willinski proposes a microwavepowered expendable upper stage vehicle with a l meter reflective dishand absorption of the focused microwave energy on a carbon absorber.Hydrogen is heated by contact with carbon absorber and fed to agimballed nozzle for guiding the vehicle. The large dish antenna isconnected to the payload by a system of wires and foam filled tubes.Microwave energy is beamed to the space vehicle by a phased array ofhigh power antennas on the ground.

U.S. Pat. No. 3.l l4,5 [7 describes a microwave powered vehicle thatapparently is restricted to use within the earths atmosphere. Thevehicle uses the atmosphere as the propellant and the highest altitudementioned is only 65,000 feet. U.S. Pat. No. 3,083,528 teaches amicrowave engine for propulsion.

It is desirable to have a microwave powered space vehicle wherein theorientation of the receiving antenna and the vehicle thrust axis can bevaried relative to each other. When this is done, the trajectory of thespace vehicle can be relatively independent of the origin of themicrowave beam. It is not necessary to orient the entire vehicle inorder to maintain the antenna pointed properly at the ground station.

It is also desirable to have a microwave powered vehicle that can remainin orbit for a prolonged period to serve as a booster for other spacevehicles. Such as tugboat" in space can effect substantial economiessince the same vehicle can be used for a number of missions.

BRIEF OF THE INVENTION There is, therefore, provided in practice of thisinvention according to a presently preferred embodiment, a microwavepowered space vehicle comprising a beam having means at one end forengaging another space vehicle and at least one propulsion nozzlemounted thereon. Propulsion fluid is fed from replaceable tanks to anabsorption chamber adjacent the nozzle. A mi crowave antenna forreceiving and concentrating a microwave beam is mounted for pivotingabout an axis transverse to the longitudinal axis of the beam. A shortwave guide from the feed horn of the antenna conveys a concentratedmicrowave beam through the pivot to the absorption chamber forabsorption by the propulsion fluid.

DRAWINGS These and other features and advantages of the presentinvention will be appreciated the same becomes better understood byreference to the following detailed description of a presently preferredembodiment when considered in connection with the accompanying drawingswherein:

FIG. 1 illustrates schematically a space tug constructed according toprinciples of this invention as it is powered by a phased array ofmicrowave antennas;

FIG. 2 is a perspective view of the microwave powered space vehicle;

FIG. 3 is a fragmentary view of the central support structure of thevehicle; and

FIG. 4 is a longitudinal cross section through an absorption chamber onthe space vehicle.

DESCRIPTION FIG. 1 illustrates schematically a technique for providingmicrowave power to an orbiting space vehicle. As illustrated in thisarrangement, a plurality of conventional microwave antennas 10 areprovided on the surface of the earth 11. These microwave antennas are ina large two-dimensional array and the microwave power applied to theseveral antennas is phase controlled for obtaining a focused microwavebeam. Such phased arrays of microwave antennas are well known for highpower radar installations. See for example, Theory and Analysis ofPhased Array Antennas, John Wiley and Sons, Inc. (1972); or R. C. HansenMicrowave Scanning Antennas, Vol. III, Array Systems, Academic Press,(1966).

A phased array extending about 7 kilometers on the surface of the earth,permits focusing of a 10 gigahertz microwave beam on a 500 meterdiameter receiving antenna at any range out to about 60,000 kilometers.The fill ratio of the phased array of 1000 l6-meter dishes is about1:]90. If one is satisfied with a shorter range, that is, if one appliespower to the microwave powered vehicle when it is relatively nearer theantennas, the base line of the phased array can be reduced. See forexample, Microwave Power Transmission from an Orbiting Solar PowerStation," Journal of Microwave Power (December, 1970). Further, byproviding a phased array of ground based antennas, very high powerlevels can be achieved since each antenna can run at a reasonable powerlevel. A total power level of 500 megawatts is deemed suitable forsubstantially any contemplated mission. With the antennas each having alow power level, conventional components for microwave generation andantenna pointing can be employed. Almost any desired power level can beachieved by varying the power from each antenna or using more or fewerground antennas without changing the width of the total antenna array.With the preferred sizes of phased array and antenna dish, the vehiclewill receive about percent of the transmitted power out to a range ofabout 60,000 kilometers.

The microwave beams 12 from the several antennas are phase locked so asto focus on the receiving antenna 13 of a microwave powered spacevehicle 14. As illustrated in FIG. I, the space vehicle 14 has a beam 17linked at one end to another space vehicle l6 for boosting it to adifferent trajectory. The general mode of operating the combinedmicrowave powered space tug l4 and payload 16 is one of intermittentlyapplying microwave power from the ground based antennas to the spacevehicle. Typically, this microwave beam is applied as the vehiclesapproach periapsis and is continued through periapsis until the vehiclesare near enough the horizon that the microwave antennas cannot properlyfocus on it. Relatively low accelerations such as (Hg or less are usedfor minimizing distortion of the large vehicle structure.

With the long range provided by a long baseline phased antenna array, itis not necessary that periapsis be in proximity to the antenna array.Any reasonably close earth approach is satisfactory Typically. power isapplied periodically during a relatively short portion of certainorbits, separated by several complete orbits of unpowered flight. Aseach increment of power is applied to the combined vehicles, theapoapsis can be increased to reach higher and higher altitudes. Orbitalcircularization at a high altitude can then be obtained by applyingpower near apoapsis.

If an escape trajectory is desired, this can be imparted to the boostedvehicle 16 and. before the micro wave powered vehicle leaves the rangeof the ground station, it is uncoupled from the boosted vehicle, rotatedl80 so that its engines are pointed in the oppo site direction. andbraked by application of microwave power to a sufficient extent that itremains in earth orbit. It can then be further braked and its orbitaltrajectory adjusted for refueling. maintenance. or picking up anotherpayload to be boosted.

A broad variety of such trajectories can be employed somewhat in thesame manner as set forth in AIAA Paper Number 72-1095, entitledReactorless Nuclear Propulsion the Laser Rocket" by Michael A. Minovitchat AlAA/SAE 8th Joint Propulsion Specialist Conference, November I972.

Referring now to FIG. 2, the microwave powered space vehicle 14 is seenin perspective. The vehicle includes a lightweight structural beam 17connected to the structure of the microwave antenna. The antennacomprises a large, lightweight parabolic dish 13 about 500 meters indiameter with a short conventional microwave feed horn 18 at the focusof the dish. The antenna is a lightweight structure assembled ordeployed in space. The parabolic dish reflector has a rigid mast 19extending aft from its convex side. A plurality of rigid masts 20 extendfrom the concave side of the dish to the central structure 21 supportingthe feed horn l8. lf desired, a single mast can be used between the dishand central support structure. A second rigid mast 22 extends beyond thesupport structure 21 to mount guidance and navigation equipment, storagebatteries, thermionic converters, communications transmitters, receiversand antennas. orientation propulsion rockets and the like designatedschematically as elements 45. If need be. some dead weight may beprovided at the end of the forward beam 22 for counterbalancing theweight of the dish 13 so that the center of mass of that portion of thevehicle rigidly connected to the dish is at the central supportstructure 21. The masts i9, 20

LII

and 22 are connected to the antenna dish l3 by multiple guy cables whichare not illustrated in the drawing to minimize confusion. The guy cablesrigidify the beams and antenna and maintain them in tension forstabilizing the entire structure. The mast may be single hollow tubesor. if need be to enhance their buckling resistance, can be in the formof open trusses. Those masts and guy cables on the forward side of theantenna dish should in general. be made of nonconductive material toavoid disturbance of the incoming or reflected microwave beams. Thegeneral structure of such a large parabolic microwave antenna stiffenedby masts and guy cables is shown and described by D. L. Pope, W. H.Hewitt, Jr., and J. G. Petz in Journal of Spacecraft and Rockets, Vol.9. No. 5, May I972. pp. 289 and 290, and as Paper Number 7l-397 at theAAS- AlAA Variable Geometry and Expandable Structures Conference. April2l-23, 1971, available from AlAA.

The central support structure 21 and adjacent elements are illustratedin greater detail in FIG. 3. The feed horn 18 is rigidly mounted in thecentral structure 21 so that it is always aligned with the parabolicantenna dish 13. A pair of wave guides 23 extend laterally from thecentral support structure and are pivotally mounted relative thereto bya conventional hollow trunnion 30. Rotating pivots for conveyingmicrowave beams are described in Radar System Fundamentals, NAVSHIPS900,017. Navy Department (1944). A short Y-shaped link 24 provides astructural connection between the wave guides 23 and the beam l7. Shortbeams 25 extend in the opposite direction from the beam 17 and providestructural support for fuel tanks 26. Preferably, the fuel tanks 26 arein the form of a cluster of replaceable tanks, one of which (260) isshown exploded from the cluster of tanks in FIG. 2. By using replaceabletanks, many of the problems of fuel transfer in vacuum and at highaltitudes beyond the range of shuttle vehicles. can be avoided.

The wave guides 23 lead to propulsion nozzles 27 by way of absorptionchambers 28 described in greater detail hereinafter.

It will be noted that the wave guides 23 with the propulsion nozzles 27are rigidly connected to the links 24 and beam 17 and also the fueltanks 26 by way of the short beams 25. This entire rigid structure isfree to pivot around the central support structure 2| about an axistransverse to the mast 22, that is, transverse to the focal axis of theantenna. It will be recalled that the center of mass of the antennasystem, including the masts and the accessory equipment 45. is locatedat the central support structure 21. The pivot axis of the wave guides23 passes through this center of mass. The thrust axes of the propulsionnozzles 27 are parallel to the beam 17 or they may be canted at a slightangle outwardly relative to each other so that their integrated thrustis along the beam 17.

During powered flight the longitudinal axis of the dish 13 must bepointed at the transmitting antenna array on the ground. For maximumefficiency the vehicles thrust vector should be aligned with thevehicles instantaneous velocity vector. This is accomplished by slowlyvarying the tilt angle 6 and slowly rotating the dish 13 about itslongitudinal axis irrespective of the vehicles trajectory. Thepropulsive thrust during these powered flight maneuvers is along thebeam 17 and always passes through the center of mass of the entirevehicle irrespective of the payload mass, fuel load, or tilt angle 0.The thrust, therefore, will never produce any unwanted torque on thevehicle. It will be noted that the antenna is spaced along the vehicleaxis well away from other components so that there is minimum shieldingof the antenna by other portions of the vehicle even when pivoted abouta substantial angle relative to the vehicle axis. The thrust axis andthe antenna axis can pivot relative to each other through angles 0 of 30to 150 or a total included angle of 120. If a somewhat smaller angle isused, some construction constraints can be relaxed.

Docking latches 29 of a conventional type are provided at the end of thebeam 17 for linking it to a payload to be boosted. (For example, theother space vehicle 16). A plurality of conventional chemical Vernierrockets (not shown) may be provided on the end of the beam and on theparabolic dish rim for assisting in controlling roll, pitch and yaw ofthe vehicle during space maneuvering. This enables trajectoryadjustments and roll control to keep the antenna properly pointed as thevehicle is operated without undue oscillating motions building up.

Hydrogen or nitrogen propellant or working fluid is stored cryogenicallyin the replaceable propulsion tanks 26 and fixed sheaths may be left onthe vehicle to provide thermal insulation and radiation protection. Thislatter is of appreciable importance when the microwave beam irradiatesthe portion of the vehicle where the hydrogen is stored, since asubstantial thermal load may be applied during that period. It will beapparent that since one uniformly maintains the antenna pointing in thedirection of the transmitting array during irradiation, radiationreflectors may be provided along that side for protecting parts of thevehicle structure from irradiation by the microwave beam.

It will be noted that the beam 17 extending between the supportingstructure 21 and the payload 16 being boosted is in tension duringacceleration. This permits a fairly long beam to keep the payload remotefrom the concentrated microwave beam irrespective of tilt angle relativeto the antenna. It also keeps the payload remote from possible damagingeffects from the rocket nozzle exhaust. Rocket exhaust is at asufficiently high velocity that little if any impingement on the payloadis encountered, particularly if the nozzles are skewed outwardly a fewdegrees. Since the payload and the fuel tanks swivel together and remainaligned with the thrust axis of the combined propulsion nozzles, vehicleperformance is not dependent on a particular payload mass or propellantquantity. It can be used for accelerating large or small payloads andcan operate with full or nearly empty propellant tanks without shift ofthe center of mass except in a direction along the thrust axis.

If desired the microwave powered space tug can be used to push" apayload rather than pull it as in the preferred embodiment. In a pushingembodiment the location of fuel tanks and payload are, in effect,switched and a shorter beam to the payload is preferred to preventbuckling. If desired the payload can be forward of the fuel tankswithout significant stability problems. With such an arrangement apropulsion nozzle on the thrust axis can be used without hazard ofexhaust impingement on other structures.

When a microwave beam strikes the antenna dish 13, it is focused on theconventional feed horn 18. This concentrated beam is collected by thefeed horn and directed into the circular wave guides 23 which carry theconcentrated beam transverse to the focal axis of the antenna. ln thisway the microwave beam is conducted through the hollow trunnions betweenthe central antenna structure 21 and the laterally extending waveguides.

The circular wave guides are preferably operated in the TE-Ol mode sincethe surface current losses are low. See, for example, Radar ElectronicFundamentals, NAVSHIPS 900,016, Navy Department (June 1944), Section 87,pages 368 to 370, in particular; or E. C. Okress, Microwave PowerEngineering, Vol. 1, (1968) Chapter 3. These surface currents yield aloss of about 0.003 db per meter in the wave guide. Since very highpowers are being transmitted through the wave guides,

forced cooling with the propulsion fluid is desirable and the cryogenicpropulsion fluid is thereby pre-heated. The wave guides are preferablyevacuated for low loss, and this is simply done by venting them to thehard vacuum of space. The circular wave guides are particularlyadvantageous for transmitting the microwave beam through the hollowtrunnions 30 of the antenna. The wave guides are simply mounted alongthe trunnion axis and no complicated mechanisms are required.

The microwave beams may be divided as desired in wave guides external tothe feed born for leading to several absorption chambers 28 ifadditional propulsion nozzles 27 are desired. It will be recognized thatthe elements 23 identified in FIG. 3 as wave guides are in actualityalso structural members for carrying the thrust of the propulsion systemto the antenna portion of the vehicle, and include propulsion fluidtransfer lines as well as the wave guides.

H6. 4 illustrates in longitudinal cross-section one of the absorptionchambers 28 connected to one of the rocket nozzles 27. A flared lowreflection horn 34 connects the end of the circular wave guide 33 to theend of the absorption chamber. A suitable dielectric material such asberyllium oxide acts as a window 36 at the output end of the transitionhorn 34. The dielectric window 36 passes the microwave beam andseparates the propulsion fluid in the absorption chamber 28 from thevacuum within the wave guide. See, for example, Harvey, Microwave Power.page 254.

The absorption chamber is about 2 meters long and formed of a dielectricmaterial such as fused quartz. The chamber has a cylindrical portion 37along most of its length and a conical portion 38 near its aft end whereit is connected to the rocket nozzle 27. The entire absorption chamberis surrounded by coils of high conductivity copper tubing 39. Theindividual turns of tubing around the absorption chamber areelectrically insulated from each other.

The working fluid or propellant is pumped through the tubing 39 and isinjected into the absorption chamher through a plurality of orifices 41around the forward end thereof. The working fluid then flows along thelength of the absorption chamber and out through the nozzle 27 forpropelling the vehicle. The microwave beam entering the absorptionchamber is absorbed by the gas in the chamber. Since high energy levelsare involved, the gas rapidly becomes a plasma which is electricallyconductive and therefore highly absorbent of the microwave beam.Generation of such a plasma may be initiated by seeding the hydrogenpropellant with a readily ionized material, such as cesium. where poweris first applied.

A heavy electric current is passed through the propellant tubing 39surrounding the absorption chamber. This current produces a magneticbottle which forces the plasma into an envelope as indicated by thephantom lines 42 in FIG. 3.

The coils of tubing surrounding the cylindrical portion 37 of thedielectric absorption chamber are of substantially uniform spacing, andmay be only one or two layers deep. In the conical portion 38 of theabsorption chamber the coils gradually become more concentrated so thatthe magnetic field intensity in the conical portion of the absorptionchamber is much higher. This increased magnetic flux is due to a greaternumber of coils and also due to the reduced cross section of thechamber. The increased magnetic field tends to pinch the plasma withinthe absorption chamber and direct the reduced cross section plasmathrough the throat of the nozzle 27 for obtaining very high exhaustvelocities.

By forcing the plasma away from the walls of the absorption chamber,heating of the walls is substantially reduced. The walls are also keptcool by the propellant flowing through the tubes 39. It will be notedthat with the copper tubes surrounding the absorption chamber it is ineffect electrically conductive so that the microwave beam is entrappedtherein. The highly absorbent plasma within the chamber absorbs most ofthe microwave energy and what little might escape appears as surfacecurrents in the copper tubing and its heat is absorbed by the flowingpropellant. Substantially quantitative absorption of the microwaveenergy and conversion to thermal energy is thereby effected. About theonly losses occurring in such an arrangement are the minor reflectionsback into the input wave guide from the window 36 and the plasma.Although this absorption chamber is preferred, other structures forconverting microwave radiation into heat for the working fluid may beused.

It turns out that about 92 percent of the power incident on the antennais available for propulsion, less whatever additional losses may beencountered due to surface imperfections in the large parabolicreflector. Some of the total energy appears as heat that is absorbed bythe propellant before it reaches the absorption chamber.

Electrical power for the magnetic coils on the absorption chamber isprovided from storage batteries 45 at the end of the mast 22. Thesebatteries are recharged by microwave powered thermionic generators thatare energized during the powered flight periods. The microwave radiationused to power these thermionic generators is received by a second butmuch smaller para bolic antenna 46 mounted at the end of mast 22. Thecombined mass of these vehicle components (although low) will contributetowards shortening the length of the counter balancing torque arm 22. Asan alternative to microwave powered theremionic generators one cangenerate electric power directly from the microwave beam by suitablerectennas located on any suitable portions of the vehicle. The antenna46 may be a rectenna for converting some of the microwave beam directlyinto electric current rather than going through the intermediate thermalstep of a thermionic generator. Rectennas have already exceeded anoperational efficiency of 80 percent at 2 gigahertz. See, for example,"The Receiving Antenna and Microwave Power Rectification" Journal ofMicrowave Power, page 279 (December l970)v This eliminates the necessityfor having thermionic generators. It will be noted that high electriccurrents are available only when the antenna is irradiated by microwaveradiation. However, these are the only times that such heavy currentsare required and, during intermediate periods during coast, powerrequirements can be satisfied by the on-hoard batteries which arerecharged during the periods of irradiation by microwave beams.

in operation the reusable microwave powered space vehicle is boostedinto low earth orbit by a conventional chemical booster or shuttlevehicle where it is assembled. Once assembled the microwave poweredvehicle stays in orbit around the earth. The working fluid tanks can beprovided by shuttle vehicle deliveries and these tanks 26 are placed inthe mounting sheaths near the vehicle center of mass. The propulsionfluid is then fed to the absorption chambers 28 as required whenmicrowave power is supplied.

A chemical powered shuttle vehicle launched from the earths surface andcarrying a payload designated for an orbit beyond its capability,rendezvouses with a microwave powered space tug. The payload istransferred and attached to the tub by docking latches 29. The tug andpayload acceleration sequence then begins via an intermittent series ofpropulsive maneuvers. After the payload is boosted into its desiredtrajectory the microwave powered vehicle is rotated 180 degrees and themain propulsion rockets 27 used for braking the vehicle and retaining itin a suitable earth orbit. lt will be noted that much less total energyis required for braking than for boosting, since by this time, thepayload is disconnected and most of the propellant is expended so thatthe total vehicle mass is relatively low. When the vehicle is broughtinto a suitable orbit any empty working fluid tanks are removed andreplaced from a space shuttle, thereby enabling the microwave poweredvehicle to conduct another mission.

Microwave power is desirable for propelling a space vehicle since theenergy can be provided from an earth station and only the propulsionfluid need be boosted into orbit. A higher specific impulse isobtainable from hydrogen than from the chemical fuels customarily used.Microwave power may be preferable to light from a laser station forseveral reasons. By using a phased array of microwave antennas the rangeof the system, that is the distance between the ground station and thespace tug, can be quite large, as compared with a laser power source. Byproperly selecting the wave length to be used (e.g. about 3 cm.), themicrowave system can be operated in any kind of weather whereas a lasersystem can not be operated in the presence of any overcast. Theefficiency of generating microwave power by way of klystron tubes is inthe range of to percent depending upon the power level and otheroperating conditions, which compares very favorably with a 20 percentefficiency in the best laser systems.

Although described hereinabove with respect to hydrogen propulsion, itwill be noted that nitrogen propulsion also has certain distinctadvantages. Nitrogen is about I! times as dense as hydrogen and can beheated with microwave radiation with substantially the same efficiency.Because of the higher density, much less massive tanks are required andsubstantial vehicle and shuttle mass savings may accrue. In addition,the temperature of liquid nitrogen is significantly higher than that ofliquid hydrogen and less sophisticated thermal insulation can beemployed. It will also be noted that liquid nitrogen is considerablyless expensive than liquid hydrogen and it can be handled with greaterease and safety. Clearly. other propulsion fluids could be employed. buthydrogen or nitrogen are presently deemed preferable.

Although limited embodiments of a reusable microwave powered spacevehicle have been described and illustrated herein, many modificationsand variations will be apparent to one skilled in the art. It is,therefore, to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A microwave powered reusable space tug compris ing:

a structural beam;

means on one end of the structural beam for engaging and propellinganother space vehicle; propulsion nozzle means on the structural beamfor propelling the tug;

an absorption chamber connected to the nozzle means;

tank means on the structural beam for containing a propulsion fluid;

means for conveying fluid from the tank means to the absorption chamber;a microwave antenna for receiving and concentrating a microwave beam;

pivot means for interconnecting the antenna and the structural beam forpivoting about an axis transverse to the longitudinal axis of the beamand keeping the antenna pointed at the microwave beam independently ofthe direction of the propulsion nozzles; and

wave guide means for conveying a concentrated microwave beam from theantenna focus through the pivot means to the absorption chamber forabsorption by the propulsion fluid.

2. A microwave powered space tug as defined in claim 1 wherein theantenna comprises a parabolic antenna dish and a feed horn; and whereinthe pivot axis extends through the center of mass of the antenna andtransverse to the focal axis of the antenna.

3. A microwave powered space tug as defined in claim 1 wherein theabsorption chamber comprises:

a dielectric housing;

means for injecting propulsion fluid into the dielectric housing; and

a conductor coil around the housing for generating a magnetic fieldwithin the housing.

4. A microwave powered space tug as defined in claim 3 wherein theconductor coil comprises tubing and wherein the propulsion fluid ispassed through the tubing prior to injection into the absorptionchamber.

5. A microwave powered space tug as defined in claim 3 wherein thedielectric housing includes a pressure resistant dielectric windowbetween the interior of the housing and the wave guide means.

6. A microwave powered space tug as defined in claim 3 wherein the tugfurther comprises a rectenna mounted thereon for generating electricpower directly from a concentrated microwave beam.

7. A microwave powered space tug comprising:

a structural beam;

a propulsion nozzle on the beam;

a microwave absorption chamber connected to the nozzle;

means for injecting a propulsion fluid into the absorption chamber;

a microwave antenna for receiving and concentrating a microwave beam;

a pair of hollow trunnions for interconnecting the antenna and thestructural beam for pivoting about an axis transverse to thelongitudinal axis of the structural beam and to the focal axis of theantenna; and

wave guide means for conveying a concentrated microwave beam from theantenna through the hollow trunnions to the absorption chamber forabsorption by the propulsion fluid.

8. A microwave powered space tug as defined in claim 7 furthercomprising:

means for replenishing fluid on the space tug while in space byinterchanging cryogenic liquid fuel tanks.

9. A microwave powered space tug as defined in claim 7 wherein theabsorption chamber comprises:

means for containing a plasma within the chamber and for directing theplasma into the nozzle.

10. A microwave powered space tug as defined in claim 9 wherein themeans for containing and directing a plasma comprises:

electrically conductive coils surrounding the absorption chamber;

means for passing a current through the coils for generating a magneticfield within the chamber; and

means for circulating propulsion fluid through the coils prior toinjection into the chamber.

1. A microwave powered reusable space tug comprising: a structural beam;means on one end of the structural beam for engaging and propellinganother space vehicle; propulsion nozzle means on the structural beamfor propelling the tug; an absorption chamber connected to the nozzlemeans; tank means on the structural beam for containing a propulsionfluid; means for conveying fluid from the tank means to the absorptionchamber; a microwave antenna for receiving and concentrating a microwavebeam; pivot means for interconnecting the antenna and the structuralbeam for pivoting about an axis transverse to the longitudinal axis ofthe beam and keeping the antenna pointed at the microwave beamindependently of the direction of the propulsion nozzles; and wave guidemeans for conveying a concentrated microwave beam from the antenna focusthrough the pivot means to the absorption chamber for absorption by thepropulsion fluid.
 2. A microwave powered space tug as defined in claim 1wherein the antenna comprises a parabolic antenna dish and a feed horn;and wherein the pivot axis extends through the center of mass of theantenna and transverse to the focal axis of the antenna.
 3. A microwavepowered space tug as defined in claim 1 wherein the absorption chambercomprises: a dielectric housing; means for injecting propulsion fluidinto the dielectric housing; and a conductor coil around the housing forgenerating a magnetic field within the housing.
 4. A microwave poweredspace tug as defined in claim 3 wherein the conductor coil comprisestubing and wherein the propulsion fluid is passed through the tubingprior to injEction into the absorption chamber.
 5. A microwave poweredspace tug as defined in claim 3 wherein the dielectric housing includesa pressure resistant dielectric window between the interior of thehousing and the wave guide means.
 6. A microwave powered space tug asdefined in claim 3 wherein the tug further comprises a rectenna mountedthereon for generating electric power directly from a concentratedmicrowave beam.
 7. A microwave powered space tug comprising: astructural beam; a propulsion nozzle on the beam; a microwave absorptionchamber connected to the nozzle; means for injecting a propulsion fluidinto the absorption chamber; a microwave antenna for receiving andconcentrating a microwave beam; a pair of hollow trunnions forinterconnecting the antenna and the structural beam for pivoting aboutan axis transverse to the longitudinal axis of the structural beam andto the focal axis of the antenna; and wave guide means for conveying aconcentrated microwave beam from the antenna through the hollowtrunnions to the absorption chamber for absorption by the propulsionfluid.
 8. A microwave powered space tug as defined in claim 7 furthercomprising: means for replenishing fluid on the space tug while in spaceby interchanging cryogenic liquid fuel tanks.
 9. A microwave poweredspace tug as defined in claim 7 wherein the absorption chambercomprises: means for containing a plasma within the chamber and fordirecting the plasma into the nozzle.
 10. A microwave powered space tugas defined in claim 9 wherein the means for containing and directing aplasma comprises: electrically conductive coils surrounding theabsorption chamber; means for passing a current through the coils forgenerating a magnetic field within the chamber; and means forcirculating propulsion fluid through the coils prior to injection intothe chamber.